Mirror Modifier isn't working correctly its offset?
Your mesh has been rotated in object mode:



To apply its rotation (i.e. making it retain its orientation while havin the angles all set to zero) use Ctrl+A. 

Read:Why is it important to apply transformation to an objects data?

When you do that, the viewport's axes will match the object internal coordinates and the Mirror modifier will work as you'd expect (you may need to change the axis you're currently using for mirroring)